The Field Effect Transistor
The transistor is a solid state semiconductor device which can be used for amplification, switching, voltage stabilization, signal modulation and many other functions. Generally, a transistor has three terminals, and a voltage applied to a specific one of the terminals controls current flowing between the other two terminals.
The terminals of a field effect transistor (FET) are commonly named source, gate and drain. In the FET a small amount of voltage is applied to the gate in order to control current flowing between the source and drain. In FETs the main current appears in a narrow conducting channel formed near (usually primarily under) the gate. This channel connects electrons from the source terminal to the drain terminal. The channel conductivity can be altered by varying the voltage applied to the gate terminal, enlarging or constricting the channel and thereby controlling the current flowing between the source and the drain.
FIG. 1 illustrates a FET 100 comprising a p-type substrate, and two spaced-apart n-type diffusion areas—one of which will serve as the “source”, the other of which will serve as the “drain” of the transistor. The space between the two diffusion areas is the “channel”. A thin dielectric layer is disposed over the substrate in the neighborhood of the channel, and a “gate” structure is disposed over the dielectric layer atop the channel. (The dielectric under the gate is also commonly referred to as “gate oxide” or “gate dielectric”.) Electrical connections (not shown) may be made to the source, the drain, and the gate. The substrate may be grounded.
Generally, when there is no voltage on the gate, there is no electrical conduction (connection) between the source and the drain. As voltage (of the correct polarity) is applied to the gate, there is a “field effect” in the channel between the source and the drain, and current can flow between the source and the drain, and can be controlled by the voltage applied to the gate. In this manner, a small signal (gate voltage) can control a relatively large signal (current flow between the source and the drain).
The Floating Gate Transistor
A floating gate transistor is generally a transistor structure, broadly based on the FET, as described hereinabove. As illustrated in FIG. 2, the floating gate transistor 200 has a source and a drain, but rather than having only one gate, it has two gates which are called control gate (CG) and floating gate (FG). It is this arrangement of control gate and floating gate which enables the floating gate transistor to function as a memory cell, as described hereinbelow.
The floating gate is disposed over tunnel oxide (comparable to the gate oxide of the FET). The floating gate is a conductor, the tunnel oxide is an insulator (dielectric material). Another layer of oxide (interpoly oxide, also a dielectric material) separates the floating gate from the control gate.
Since the floating gate is a conductor, and is surrounded by dielectric material, it can store a charge. Electrons can move around freely within the conductive material of the floating gate (which comports with the basic definition of a “conductor”).
Since the floating gate can store a charge, it can exert a field effect on the channel region between the source and the drain, in a manner similar to how a normal FET works, as described hereinabove. Mechanisms for storing charges on the floating gate structure, as well as removing charges from the floating gate are described hereinbelow.
Generally, if a charge is stored on the floating gate, this represents a binary “1”. If no charge is stored on the floating gate, this represents a binary “0”. (These designations are arbitrary, and can be reversed so that the charged state represents binary “0” and the discharged state represents binary “1”.) That represents the programming “half” of how a floating gate memory cell operates. The other half is how to determine whether there is a charge stored on the floating gate—in other words, to “read” the memory cell. Generally, this is done by applying appropriate voltages to the source, drain and gate terminals, and determining how conductive the channel is. Some modes of operation for a floating gate memory cell are described hereinbelow.
Normally, the floating gate non-volatile memory (NVM) cell has only a single “charge-storing area”—namely, the conductive floating gate (FG) structure, and can therefore only store a single bit of information (binary “1” or binary “0”). More recently, using a technology referred to as “multi-level cell” (MLC), two or more bits can be stored in and read from the floating gate cell.
The NROM Memory Cell
Another type of memory cell, called a “nitride, read only memory” (NROM) cell, has a charge-storage structure which is different from that of the floating gate memory cell and which permits charges to be stored in two separate charge-storage areas. Generally, the two separate charge storage areas are located within a non-conductive layer disposed between the gate and the underlying substrate, such as a layer of nitride formed in an oxide-nitride-oxide (ONO) stack underneath the gate. The non-conductive layer acts as a charge-trapping medium. Generally, electrical charges will stay where they are put in the charge-trapping medium, rather than being free to move around as in the example of the conductive floating gate of the floating gate memory cell. A first bit of binary information (binary “1” or binary “0”) can be stored in a first portion (such as the left-hand side) of the charge-trapping medium, and a second bit of binary information (binary “1” or binary “0”) can be stored in a second portion (such as the right-hand side) of the charge-trapping medium. An alternative viewpoint is that different charge concentrations can be considered for each bit of storage. Using MLC technology, at least two bits can be stored in and read from each of the two portions (charge storage areas) of the charge-trapping medium (for a total of 4 bits), similarly 3 bits or more than 4 bits may be identified.
FIG. 3 illustrates a basic NROM memory cell, which may be viewed as an FET with an “ONO” structure inserted between the gate and the substrate. (One might say that the ONO structure is “substituted” for the gate oxide of the FET.)
The ONO structure is a stack (or “sandwich”) of lower oxide 322, a charge-trapping material such as nitride 324, and an upper oxide 326. The ONO structure may have an overall thickness of approximately 10-25 nm, such as 18 nm, as follows:                the bottom oxide layer 322 may be from 3 to 6 nm, for example 4 nm thick;        the middle nitride layer 324 may be from 3 to 8 nm, for example 4 nm thick; and        the top oxide layer 326 may be from 5 to 15 nm, for example 10 nm thick.        
The NROM memory cell has two spaced apart diffusions 314 and 316 (which can function as source and drain, as discussed hereinbelow), and a channel region 320 defined in the substrate between the two diffusion regions 314 and 316.
In FIG. 3, the diffusions are labeled “N+” (compare FIG. 1, n-type). This means that they are regions in the substrate that have been doped with an electron donor material, such as phosphorous or arsenic. These diffusions are typically created in a larger region which is p-type cell well (CW) is doped with boron (or indium). This is the normal “polarity” for a NVM cell employing electron injection (which may also employ hole injection, such as for erase). With opposite polarity (boron or indium implants in a n-type cell well), the primary injection mechanism would be for holes, which is generally accepted to be not as effective as electron injection. One skilled in the art will recognize that the concepts disclosed herein can be applied to opposite polarity devices.
The charge-trapping material 324 is non-conductive, and therefore, although electrical charges can be stored in the charge-trapping material, they are not free to move around, they will generally stay where they are stored. Nitride is a suitable charge-trapping material. Charge trapping materials other than nitride may also be suitable for use as the charge-trapping medium. One such material is silicon dioxide with buried polysilicon islands. A layer (324) of silicon dioxide with polysilicon islands would be sandwiched between the two layers of oxide (322) and (326). Alternatively, the charge-trapping layer 324 may be constructed by implanting an impurity, such as arsenic, into a layer of silicon dioxide deposited on top of the bottom oxide 322.
The memory cell 300 is generally capable of storing at least two bits of data—at least one bit(s) in a first storage area of the nitride layer 324 represented by the dashed circle 323, and at least one bit(s) in a second storage area of the nitride layer 324 represented by the dashed circle 321. Thus, the NROM memory cell can be considered to comprise two “half cells”, each half cell capable of storing at least one bit(s). It should be understood that a half cell is not a physically separate structure from another half cell in the same memory cell. The term “half cell”, as it may be used herein, is used herein only to refer to the “left” or “right” bit storage area of the ONO stack (nitride layer). The storage areas 321, 323 may variously be referred to as “charge storage areas”, “charge trapping areas”, and the like, throughout this document. (The two charge storage areas may also be referred to as the right and left “bits”.)
Each of the storage areas 321, 323 in the charge-trapping material 324 can exert a field effect on the channel region 320 between the source and the drain, in a manner similar to how a normal FET works, as described hereinabove (FIG. 2).
Generally, if a charge is stored in a given storage area of the charge-trapping material, this represents a binary “1”, and if no charge is stored in a given storage area of the charge-trapping material, this represents a binary “0”. (Again, these designations are arbitrary, and can be reversed to that the charged state represents binary “0” and the discharged state represents binary “1”.) That represents the programming “half” of how an NROM memory cell operates. The other half is how to determine whether there is a charge stored in a given storage area of the charge-trapping material—in other words, to “read” the memory cell. Generally, this is done by applying appropriate voltages to the diffusion regions (functioning as source and drain) and gate terminals, and determining how conductive the channel is.
Generally, one feature of NROM cells is that rather than performing “symmetrical” programming and reading, NROM cells are beneficially programmed and read “asymmetrically”, which means that programming and reading occur in opposite directions. The arrows labeled in FIG. 3 are arranged to illustrate this point. Programming may be performed in what is termed the “forward” direction and reading may be performed in what is termed the “opposite” or “reverse” direction.
“Reading” an NROM Cell
Reading an NROM memory cell may involve applying voltages to the terminals of the memory cell comparable to those used to read a floating gate memory cell, but reading may be performed in a direction opposite to that of programming. Generally, rather than performing “symmetrical” programming and reading (as is the case with the floating gate memory cell, described hereinabove), the NROM memory cell is usually programmed and read “asymmetrically”, meaning that programming and reading occur in opposite directions. This is illustrated by the arrows in FIG. 3. Programming is performed in what is termed the forward direction and reading is performed in what is termed the opposite or reverse direction. For example, generally, to program the right storage area 323 (in other words, to program the right “bit”), electrons flow from left (source) to right (drain). To read the right storage area 323 (in other words, to read the right “bit”), voltages are applied to cause electrons to flow from right to left, in the opposite or reverse direction. For example, generally, to program the left storage area 321 (in other words, to program the left “bit”), electrons flow from right (source) to left (drain). To read the left storage area 321 (in other words, to read the left “bit”), voltages are applied to cause electrons to flow from left to right, in the opposite or reverse direction. See, for example, U.S. Pat. No. 6,768,165.
Memory Array Architecture Generally
Memory arrays are well known, and comprise a plurality (many, including many millions) of memory cells organized (including physically arranged) in rows (usually represented in drawings as going across the page, horizontally, from left-to-right) and columns (usually represented in drawings as going up and down the page, from top-to-bottom).
As discussed hereinabove, each memory cell comprises a first diffusion (functioning as source or drain), a second diffusion (functioning as drain or source) and a gate, each of which has to receive voltage in order for the cell to be operated, as discussed hereinabove. Generally, the first diffusions (usually designated “source”) of a plurality of memory cells are connected to a first bit line which may be designated “BL(n)”, and second diffusions (usually designated “drain”) of the plurality of memory cells are connected to a second bit line which may be designated “BL(n+1)”. Typically, the gates of a plurality of memory cells are connected to common word lines (WL).
The bitlines may be “buried bitline” diffusions in the substrate, and may serve as the source/drain diffusions for the memory cells. The wordlines may be polysilicon structures and may serve as the gate elements for the memory cells.
FIG. 4 illustrates an array of NROM memory cells (labeled “a” through “i”) connected to a number of word lines (WL) and bit lines (BL). For example, the memory cell “e” has its gate connected to WL(n), its source (left hand diffusion) is connected to BL(n), and its drain (right hand diffusion) is connected to BL(n+1). The nine memory cells illustrated in FIG. 4 are exemplary of many millions of memory cells that may be resident on a single chip.
Notice, for example that the gates of the memory cells “e” and “f” (to the right of “e”) are both connected to the same word line WL(n). (The gate of the memory cell “d” to the left of “e” is also connected to the same word line WL(n).) Notice also that the right hand terminal (diffusion) of memory cell “e” is connected to the same bit line BL(n+1) as the left-hand terminal (diffusion) of the neighboring memory cell “f”. In this example, the memory cells “e” and “f” have two of their three terminals connected together.
The situation of neighboring memory cells sharing the same connection—the gates of neighboring memory cells being connected to the same word line, the source (for example, right hand diffusion) of one cell being connected to the drain (for example left hand diffusion) of the neighboring cell—is even more dramatically evident in what is called “virtual ground architecture” wherein two neighboring cells actually share the same diffusion. In virtual ground array architectures, the drain of one memory cell may actually be the same diffusion which is acting as the source for its neighboring cell. Examples of virtual ground array architecture may be found in U.S. Pat. Nos. 5,650,959; 6,130,452; and 6,175,519, incorporated in their entirety by reference herein.
Modes of Operation
Generally, the modes of operation for any NVM memory cell (either floating gate or NROM) include “program”, “erase” and “read”. Modes of operation for NROM are now discussed.
Program generally involves injecting electrons into the charge storage areas of the NROM cell, typically by a process known as channel hot electron (CHE) injection. Exemplary voltages to program (by CHE injection of electrons) the right bit (right bit storage area) of an NROM cell,                the left BL (acting as source, Vs) is set to 0 volts        the right BL (acting as drain, Vd) is set to +5 volts        the gate (Vg) is set to +8-10 volts        the substrate (Vb) is set to 0 voltsand the bit storage area above the drain (right BL) becomes programmed. To program the left bit storage area, source and drain are reversed—the left bitline serves as the drain and the right bitline serves as the source.        
Erase may involve injecting holes into the charge storage areas of the NROM cell, typically by a process known as hot hole injection (HHI). Generally, holes cancel out an electrons (they are electrically opposite), on a one-to-one basis. Exemplary voltages to erase (by HHI injection of holes) the right bit of the NROM cell,                the left BL (acting as source, Vs) is set to float        the right BL (acting as drain, Vd) is set to +5 volts        the gate (Vg) is set to −7 volts        the substrate (Vb) is set to 0 voltsand the bit storage area above the drain (right BL) becomes erased. To erase the left bit storage area, source and drain are reversed—the left bitline serves as the drain and the right bitline serves as the source.        
Read involves applying voltages to the terminals of the memory cell and, based on subsequent current flow, ascertaining the threshold voltage of the charge storage area within the cell. Generally, to read the right bit of the NROM cell, using “reverse read”,                the right BL (acting as source, Vs) is set to 0 volts        the left BL (acting as drain, Vd) is set to +2 volts        the gate (Vg) is set to +5 volts        the substrate (Vb) is set to 0 voltsand the bit storage area above the source (right BL) can be read. To read the left bit storage area, source and drain are reversed—the left bitline serves as the source and the right bitline serves as the drain.Reading the State of the Memory Cells        
A memory cell may be programmed to different states, or program levels, determined by the threshold voltage (Vt) of the cell. For a single level cell (SLC), there are two program levels, generally “erase” and “program”. For a multi-level cell (MLC) there are more than two program levels. An NVM cell's state may be defined and determined by its threshold voltage (Vt), the voltage at which the cell begins to conduct current. A NVM cell's threshold voltage level is usually correlated to the amount of charge stored in a charge storage region of the cell. Different threshold voltage ranges are associated with different states or program levels of an NVM cell.
Generally, in order to determine the state (program level) of an NVM cell, the cell's threshold level may be compared to that of a reference structure or cell whose threshold level is set, or otherwise known to be, at a voltage level associated with the specific state being tested for. Comparing the threshold voltage of a NVM cell to that of a reference cell is often accomplished using a sense amplifier or similar circuit. Various techniques for comparing an NVM cell's threshold voltage against those of one or more reference cells or structures, in order to determine the NVM cell's state, are well known.
When reading a NVM cell, to determine whether it is at a particular state, the cell's threshold voltage may be compared against that of a reference cell having a reference threshold voltage defined as a “read” level for the specific state. A “read” level is usually set lower than a program verify (PV) level and higher than the erase verify (EV) level in order to compensate for voltage drifts which may occur during operation.
In a “binary” or single level cell (SLC) capable of storing only one bit of information (a logic 1 or a logic 0), only a single read verify (RV) voltage is required, and it may be between the erase verify (EV) and program verify (PV) voltages for the cell.
FIG. 5A is a graph illustrating two states of a “binary” or single level cell (SLC) capable of storing one bit of information per cell (or per charge storage area with an NROM cell), and utilizes only one read verify threshold (RV). Generally, the two states are erased (represented by “1”) and programmed (represented by “0”). The horizontal axis is threshold voltage (Vt), increasing from left to right.
Three voltage levels are illustrated in FIG. 5A, these are EV (erase verify), RV (read verify) and PV (program verify). As illustrated, EV is less than RV which is less than PV. A high VT may represent a program state of binary “0”, and a low Vt may represent an erase state of binary “1”. The binary designations are arbitrary, and may be reversed (high Vt=“1”, low Vt=“0”).
FIG. 5A is generalized, and is applicable to a typical floating gate NVM memory cell or a given charge storage area of an NROM cell. The curves represent the threshold voltages (Vts) for a number of cells at the given program level. Typically, there is a distribution, or spread, about a nominal (or average, or center) value. For example,                the center value for “1” equals approximately 3.5 volts        the center value for “0” equals approximately 6.0 volts        EV equals approximately 4.0 volts        RV equals approximately 4.5 volts        PV equals approximately 5.5 volts        
FIG. 5B illustrates a situation wherein there are four possible MLC program levels (or states) 11, 01, 00, 10 for each memory cell (or, in the case of NROM, for each storage area of the memory cell). As illustrated, the program level 11 has the lowest Vt, the program level 01 has a higher Vt, the program level 00 has a yet higher Vt, and the program level 10 has a yet higher Vt. The program level 11 may be erase (ERS), which for purposes of this discussion is considered to be a program level, although it is not generally regarded as such.
There are a number of memory cells (or storage areas NROM cells) being programmed, erased and read. In a given array, or on a given memory chip, there may be many millions of memory cells. Programming may typically be performed in blocks, of thousands of memory cells. The different blocks of memory cells are typically located at different logical positions within the array, and at different physical positions on the chip. During (or before) programming, a checksum indicative of the number of cells programmed to each level may be stored, in the block, in the array, on the chip, or external to the chip.
At each program level (and this is also true for the SLC cell of FIG. 5A), there is typically a distribution of threshold voltages, within a range (a statistical spread). In other words, for a given program level, the threshold voltage is not likely to be exactly a unique, precise voltage for all of the memory cells being programmed to that level. Initially, in the act of programming the cell, the voltage may be off a bit, for example as a result of the state of neighboring cells (or the other charge storage area in the same NROM cell). Or, as a result of previous program or erase operations on the same cell, or neighboring cells. Or, as a result of a variety of other factors. And, after programming, the threshold voltage of a cell may change, as a result of programming neighboring cells (or the other charge storage area in the same NROM cell), or a variety of other factors.
Therefore, the threshold voltage (Vt) for a given program level may be more than average in some cells, in others it may be less than average. Nevertheless, in a properly functioning group of cells (such as a block, or an array), there should be a clear distribution of four distinct program levels, such as illustrated. And, the distributions of Vt for each of the program levels should be separated enough from one another so that read positions (RV voltage levels) can be established between adjacent distributions of threshold voltages, such as the following:    RV01 is between EV and PV01, or higher than the highest expected Vt for a cell at state “11” and lower than the lowest expected Vt for a cell at state “01”;    RV00 is between PV01 and PV00, or higher than the highest expected Vt for a cell at state “01” and lower than the lowest expected Vt for a cell at state “00”; and    RV10 is between PV00 and PV10, or higher than the highest expected Vt for a cell at state “00” and lower than the lowest expected Vt for a cell at state “10”.
For example,                the center value for “11” equals approximately 4.0 volts        the center value for “01” equals approximately 4.4 volts        the center value for “00” equals approximately 4.8 volts        the center value for “10” equals approximately 5.4 volts        EV equals approximately 4.0 volts        RV01 equals approximately 4.4 volts        PV01 equals approximately 4.8 volts        RV00 equals approximately 5.4 volts        PV00 equals approximately 5.6 volts        RV10 equals approximately 6.0 volts        PV10 equals approximately 6.3 voltsAn Aside about Binary Notation, and the Labeling of Program Levels        
“Binary” generally means “two”. In binary notation, there are only two possible digits, usually referred to as “1” and “0”. Many 1s and 0s can be strung together to represent larger numbers, for example:                0000 is zero        0001 is one        0010 is two        0011 is three        0100 is four        1000 is eight        1010 is ten        
In the examples above, the binary numbers have four digits each—four “places”. For purposes of this disclosure, only two digits will be used. Two digits can represent four numbers. Counting (in binary) typically starts with zero, and counting from zero to three proceeds like this: 00 (zero), 01 (one), 10 (two), 11 (three). Notice, in the transition from 01 (one) to 10 (two), both bits change.
Since it is arbitrary, which program levels represent which digits, notice in FIG. 5B that the program levels appear to be out of sequence, starting with 11 (three), then 01 (one), then 00 (zero), then 10 (two). This sequence is common so that when moving from one program level to the next higher level, both bits do not change—as is the case with the transition from 01 (one) to 10 (two). In FIG. 5B it can be seen that when moving from one program level to another, only one of the bits changes.
Threshold Voltage Drift
The threshold voltage of a NVM cell seldom stays fixed (after it is programmed, or erased). Threshold voltage drift is a phenomenon which may result in large variations of the threshold voltage of a memory cell. These variations may occur due to charge leakage from the cell's charge storage region, temperature changes, and due to interference from the operation of neighboring NVM cells.
The drift in threshold voltage of a memory cell is well known, and is discussed for example in U.S. Pat. Nos. 6,992,932 and 6,963,505 discloses read error detection in a NVM array, and may hereinafter be referred to as the “moving reference patent(s)”. These deviations in a cell's threshold voltage (Vt) may be either in the upward or downward direction, and may vary from cell to cell.
Variation of the threshold voltage of memory cells may lead to false reads of the cell's state and may further result in the corruption of the data in the memory array. Voltage drift is especially problematic in MLC cells (see FIG. 5A) where the Vt regions or sub-ranges associated with each programmed state are relatively smaller than those for a typical binary or SLC cell (see FIG. 5B).
It is known that, in order to reduce data loss and data corruption due to drift in the threshold voltages of the cells of a NVM array, threshold voltage drift of cells in the NVM array should be compensated for.
The moving reference patents disclose that, for a given NVM array, it is known to provide one or a set of reference cells whose references threshold voltages are offset from defined verify threshold levels by some value related to the actual voltage drift experienced by the NVM cells to be read. There is a well understood need for an efficient and reliable method of determining a set of reference voltage levels which may accommodate variations in the threshold voltages of cells of an NVM array, and of established reference cells with the determined reference voltages.
Generally, at least a subset of cells of a NVM block (or array) may be read, and the number of cells found at a given state (such as logic “0”, or “00”) associated with the block may be compared to one or more check sum values obtained during programming of the at least a subset of cells. A Read Verify threshold reference voltage associated with the given program state or associated with an adjacent state may be adjusted based on the result of the comparison.
Generally, the idea presented in the aforementioned moving reference patents is to select (establish) a set of reference cells (from N sets) to be used in operating an NVM block or array. For example, each set of test reference cells may have reference voltages at least slightly offset from each other set of test reference cells. For example, each set of test reference cells may be incrementally offset, such that each set may be associated with a series of threshold voltages that are slightly higher than a corresponding series of threshold voltages associated with the previous set of test reference cells (excluding the first set). As a further example, if the first set of test reference cells includes cells having reference voltages; Cell 1=4.2V, Cell 2=5.2V, Cell 3=6.2V, the second set may include cells having reference voltages offset, such that; Cell 1=4.3V, Cell 2=5.3V, Cell 3=6.3V.
A set of reference voltages associated with the selected test set may be obtained by a controller. The set of reference voltages may be recorded, for example in an error rate table. The controller may instruct an offset circuit to offset the threshold voltages of one or more of the reference cells in a set of global reference cells in accordance with the set of reference voltages. The controller may instruct the offset circuit to offset the reference voltages of one or more of the global reference cells in the set of global reference cells, such that the threshold voltages of the set of global reference cells may be substantially equal to the threshold voltages of the selected test set.
The offset circuit and the set of global reference cells may be substituted with a bank of reference cells (not shown). The bank of reference cells may include two or more reference cells each reference cell in the bank being incrementally offset from the other reference cells in the bank. For example, each reference cell in the bank may have a threshold voltage that is slightly higher than the threshold voltage of the previous reference cell (excluding the first reference cell).
Once selected, the selected set of test reference cells may be used to determine which of the reference cells in the bank of reference cells is to be used for establishing an operating set of reference cells. The selected set of reference cells from the bank of reference cells may be selected such that the selected set from the bank may have reference voltages that are substantially equal to those of the selected test set. Thus, the selected set of reference cells from the bank may provide a set of operating reference cells having reference voltages substantially equal to those of the selected test set. The set of operating reference cells may be used to operate the NVM array.
Determining that Shifting RV is Necessary
Prior to or during the programming of a set of cells in a NVM array, the number of cells to be programmed to each of one or more logical or program states associated with the set of cells may be counted, and may be stored, for example in a check sum table. The number of cells to be programmed to, up to and/or below each logical or program state may be counted and/or stored in a table which is either on the same array as the set of NVM cells or in memory on the same chip as the NVM array.
Upon the reading of the set of programmed cells, the number of cells found to be at a given logical or program state may be compared against either corresponding values stored during programming (such as the number of cells programmed to a given state) or against a value derived from the values stored during programming (such as the number of cells programmed at or above the given state, minus the number of cells programmed to or above an adjacent higher logical state).
If there is a discrepancy between the number of cells read at a given state and an expected number based on the values determined/counted/stored during programming, a Read Verify reference threshold value associated with the given program state may be adjusted upward or downward to compensate for the detected error. The read verify level of an adjacent logical state may also be moved upward or downward in order to compensate for detected read errors at a given state.
For example, if the number of cells found (read) in a given program state is below an expected value, either the Read Verify reference voltage associated with that given state may be reduced, or if there is found that the number of cells read above the given state exceeds an expected number, the Read Verify reference associated with a logical state higher and adjacent to the given state may be raised. Conversely, if the number of cells found. (e.g. read) in a given program state is above expectations, either the Read Verify reference voltage associated with that given state may be increased, or if there is found that the number of cells read above the given state is below an expected number, the Read Verify reference associated with a logical state higher and adjacent to the given state may be lowered. Thus, Read Verify reference voltages for a set of cells may be selected such that the number of cells found/read in each of the states associated with the set may be substantially equal to the a number either read from or derived from the values counted during programming of the set of cells, which values may have been stored in a check sum table.
The check sum table may reside on the same chip as the set of NVM cells, and a controller may be adapted to perform the above mentioned error detection and Read Verify reference value adjustments. The check sum table may either be stored in the same NVM array as the set of NVM cells, or on some other memory cells residing on the same chip as the NVM array, for example in a register or buffer used by the controller during programming and/or reading. Specialized error coding and detection circuits may be included with a controller on the same chip and the NVM array to be operated.
During the reading of the cells from the programmed set of cells, either the controller or some other error detection circuit may compare the number of cells counted in each program state during reading with the correspond check sum values stored during or prior to programming. For example, if the number of cells found in a given program state exceed the value derived from the check sum values, the read verify (RV) threshold value associated with that given program state may be raised or the Read Verify reference level associated with the adjacent higher state may be lowered. Conversely, if the number of cell's found in a given program state is below the expected number, either the read verify threshold value associated with the given program state may be lowered, or the read verify threshold value associated with the next higher adjacent state may be raised.
If the number of cells found (read) in a given program state is below an expected value, either the Read Verify reference voltage associated with that given state may be reduced, or if there is found that the number of cells read above the given state exceeds an expected number, the Read Verify reference associated with a logical state higher and adjacent to the given state may be raised. Conversely, if the number of cells found (e.g. read) in a given program state is above expectations, either the Read Verify reference voltage associated with that given state may be increased, or if there is found that the number of cells read above the given state is below an expected number, the Read Verify reference associated with a logical state higher and adjacent to the given state may be lowered.
Thus, Read Verify reference voltages for a set of cells may be selected such that the number of cells found/read in each of the states associated with the set may be substantially equal to the a number either read from or derived from the values counted during programming of the set of cells, which values may have been stored in a check sum table.
The steps described hereinabove may be repeated as part of an iterative process until the number of cells read in each program state substantially corresponds to the number of cells expected in each state based on data recorded during programming. The process may start with the checking of cells programmed to the highest logical state, or cells programmed to several different states may be checked in parallel.
Commonly-owned patents disclose structure and operation of NROM and related ONO memory cells. Some examples may be found in commonly-owned U.S. Pat. Nos. 5,768,192 and 6,011,725, 6,649,972 and 6,552,387.
Commonly-owned patents disclose architectural aspects of an NROM and related ONO array, (some of which have application to other types of NVM array) such as segmentation of the array to handle disruption in its operation, and symmetric architecture and non-symmetric architecture for specific products, as well as the use of NROM and other NVM array(s) related to a virtual ground array. Some examples may be found in commonly-owned U.S. Pat. Nos. 5,963,465, 6,285,574 and 6,633,496.
Commonly-owned patents also disclose additional aspects at the architecture level, including peripheral circuits that may be used to control an NROM array or the like. Some examples may be found in commonly-owned U.S. Pat. Nos. 6,233,180, and 6,448,750.
Commonly-owned patents also disclose several methods of operation of NROM and similar arrays, such as algorithms related to programming, erasing, and/or reading such arrays. Some examples may be found in commonly-owned U.S. Pat. Nos. 6,215,148, 6,292,394 and 6,477,084.
Commonly-owned patents also disclose manufacturing processes, such as the process of forming a thin nitride layer that traps hot electrons as they are injected into the nitride layer. Some examples may be found in commonly-owned U.S. Pat. Nos. 5,966,603, 6,030,871, 6,133,095 and 6,583,007.
Commonly-owned patents also disclose algorithms and methods of operation for each segment or technological application, such as: fast programming methodologies in all flash memory segments, with particular focus on the data flash segment, smart programming algorithms in the code flash and EEPROM segments, and a single device containing a combination of data flash, code flash and/or EEPROM. Some examples may be found in commonly-owned U.S. Pat. Nos. 6,954,393 and 6,967,896.
A more complete description of NROM and similar ONO cells and devices, as well as processes for their development may be found at “Non Volatile Memory Technology”, 2005 published by Saifun Semiconductor and materials presented at and through http://siliconnexus.com, both incorporated by reference herein in their entirety.